I got 19.3 working on an Axon 2017G with raystef66's Slim B03 ....
Steps :
Install Nfound' s TWRP 3.2.1.8 Axon 7 Oreo.
From within TWRP,install raystef66's B03 bootstack.
Install Slim B03.
Run Raystef66's A2017G_O_system_permission_enable patch.
Install Magisk 19.3.
reboot (don't clear cache and dalvik)
After the reboot,you'll get an error about a damaged sdcard,if there's 1 installed .....ATM,I didn't found a way to format it as removable storage,because the format doesn't work,no matter what filesystem is used for the refreshing.
However,formatting the sdcard as extra internal storage works,and is recognised by the systm,so I can live with that...

Reboot,go to TWRP ,clear cache and Dalvik and you're good to
[email protected] least,I got so far with this approach....